1. Field of the Invention
This invention broadly relates to archwires that are used during orthodontic treatment. More particularly, this invention concerns orthodontic archwires having a reduced stiffness while also providing good control over movement of the associated teeth.
2. Description of the Related Art
Orthodontic treatment involves movement of maloccluded teeth to orthodontically correct positions. In many types of treatment, tiny slotted devices known as brackets are fixed to the patient's teeth, and a resilient archwire is inserted in the slot of each bracket. The archwire serves as a track to guide movement of the brackets so that the associated teeth are moved to desired positions.
Many commonly available orthodontic brackets have a slot with a rectangular cross-sectional configuration. The rectangular shape of the slot is adapted to mate with archwires having rectangular configurations in longitudinally transverse cross-sections. The matching, rectangular shapes of the slot and the archwire serve to non-rotatably couple each bracket to the archwire. As a consequence, the orthodontist can, if desired, twist or bend the archwire between adjacent teeth in order to impose a torquing or uprighting force on the teeth as may be needed to correct the occlusion of a particular patient.
Typically, only a single set of brackets is affixed to the patient's teeth during the course of treatment. However, the archwire may be changed at selected intervals in the treatment program and replaced with another archwire having somewhat different characteristics. For example, an archwire having a relatively low stiffness may be used initially when the teeth are located some distance from their intended final positions so that undue and possibly painful forces are not experienced by the patient. As the teeth move closer to their desired final positions, the archwire is often replaced with an archwire having a higher stiffness in order to facilitate moving the teeth over the remaining distances.
In some treatment programs, an archwire having a round cross-sectional configuration and a relatively low stiffness is used during the initial stage of treatment. Round archwires with relatively low stiffness are often used when the teeth are initially severely maloccluded, since these archwires offer little resistance to bending and can be ligated to each bracket without significant force. For example, when a pair of adjacent teeth are located labial-lingually with respect to each other (i.e., in directions along a line extending from the lips or cheeks to the tongue), low stiffness round archwires are often deemed satisfactory for moving such teeth closer together without causing undue pain to the patient. Unfortunately, round archwires can rotate in the rectangular slots of the brackets and therefore do not allow the orthodontist to apply a torquing or uprighting force as may be needed on selected teeth by placing bends or twists in the archwires.
It has been suggested in the past to modify the cross-sectional configuration of orthodontic archwires for one reason or another. For example, U.S. Pat. No. 5,468,147 describes an archwire having a substantially rectangular cross-section with longitudinal grooves extending along each of the four exterior surfaces of the archwire in order to increase the flexibility of the archwire. Orthodontic archwires with other cross-sectional configurations are described in U.S. Pat. Nos. 5,474,447, 5,456,599, 5,174,753 and 4,850,865.
In recent years, there has also been increased interest in the use of aesthetic orthodontic brackets that tend to minimize the appearance of metal in the oral cavity. For example, ceramic orthodontic brackets have now been developed that are translucent and assume the color of the underlying tooth. Translucent plastic brackets and tooth-colored plastic brackets are also known.
The use of aesthetic orthodontic brackets can present a significantly improved appearance in the oral cavity. Oftentimes, the archwire is the only metal component that is readily visible. Consequently, it would be desirable to reduce or eliminate this last remaining metallic appearance at least from anterior teeth if possible.
Orthodontic archwires that are coated with a non-metallic aesthetic layer have been proposed in the past. For example, U.S. Pat. No. 5,454,716 and PCT published application No. WO 97/29712 describe orthodontic archwires that are coated with a thin coloring layer that matches the color of the teeth. Other coated orthodontic archwires are described in U.S. Pat. Nos. 4,050,156 and 3,504,438. U.S. Pat. No. 4,731,018 describes an archwire with a metal part and a plastic part arranged so that the plastic part faces in a labial direction.